Certain periodic signals are used as messages or report markers or state characterizing markers in data transmission units and data processing units. Interpretation circuits provided for these characteristic signals or markers are thus to be examined for their response by adjusting the suitable signal at a send-side word generator. A receiver synchronized to the sending signal generates the same word and compares the generated word with the word received via the transmission path. Devices operating under these principles are known.
With respect to a comparison at the receiver-side, similarly built bit error measuring devices are known which utilize pseudorandom binary sequences (PRBS) as test signals. A PRBS of a length (2.sup.r -1) bit contains periodic partial sequences only in very short sections of below a 2r bit length. During operation of the transmission paths to be tested, longer periodic sequences may be obtained in lightly allocated multiplex systems. Despite certain counter measures, like scrambling, coding, monitoring of the periodicity, difficulties were encountered in some transmission paths when the input signals were periodic over a longer period. Therefore, known bit-error-measuring devices include a generator for periodic partial sequences which are separately to be switched as e.g. the measuring device PF4 of Wandel and Goltermann GmbH & Co., Eningen U.A. bei Rentlingen, Germany.
The content of the n-bit word representing a period of the periodic signal is set in known word generators and bit-error measuring devices at the start of the measurement and if necessary is electronically stored. This, however, has two drawbacks. Known data transmission devices and data processing devices use certain signals as messages or characteristic states. Although the response of these circuits can be tested in case the matched periodic signal is set, an unintentional defective response of such a circuit to another periodic signal will, however, not be recognized as the variety for a manual setting is too high. Testing of all 2.sup.n possible n-bit words is already time consuming when n=8. With n=16, such testing is more or less impractical.
The second drawback arises for the following reason. The measuring result depends frequently not only on the input signal but also on the initial state of the measuring object. Transmission paths are generally organized in their circuit configuration according to exponents of 2 (binary counter, multiplexer, framing structure). Thus, when a periodic signal of a period length of 8 bit or 16 bit is continuously transmitted, the same initial state is always obtained. Even if the critical one of the 2.sup.n possible n-bit words is coincidently set, the searched-for error function is obtained only if coincidently the matched initial state of the system existed. Thus, it is almost impossible to find a critical periodic signal by systematic measurements.